REMOVE-DUPLICATES

Remove-duplicates returns the result of deleting duplicate
elements from the beginning of the given string or true list, i.e.,
leaving the last element in place. For example,

(remove-duplicates '(1 2 3 2 4))

is equal to '(1 3 2 4).

The guard for Remove-duplicates requires that its argument is a
string or a true-list of eqlablep objects. It uses the function
eql to test for equality between elements of its argument.

Remove-duplicates is a Common Lisp function. See any Common Lisp
documentation for more information. Note that we do not allow
keyword arguments (such as test) in ACL2 functions, in
particular, in remove-duplicates. But
see remove-duplicates-equal, which is similar but uses the
function equal to test for duplicate elements.